The present invention comprehends an apparatus for spreading and pushing material onto a substrate, and more particularly pertains to an adjustable blade for spreading a viscous liquid or paste-like material onto printed circuit boards and other substrates.
In the surface mount technology field and hybrid microelectronics field, manual, semi-automatic, or automatic screen and stencil printing equipment is typically used for printing solder paste, epoxies (both conductive and adhesive), conductive inks, resistive inks, and dielectric inks, for example, onto printed circuit boards and other substrates.
The circuit board, or other substrate, is placed on the stencil or screen printer, and then one of the above-mentioned materials is printed onto the circuit board through either a metal mask stencil with open apertures or through a template having a plurality of interstices. In addition, the materials can be printed onto a circuit board through a cross-sectional screen mesh that has an emulsion in it, and the circuit or pattern to be printed on the circuit board is etched out of the emulsion. The various above-described materials are used for the purpose of attaching electrical components to the substrate and for manufacturing electrical circuits on the substrate.
The typical screen or stencil printer includes a tooling plate upon which the circuit board is positioned and aligned, a squeegee assembly or squeegee head assembly, and a metal or polyurethane blade referred to in the industry as a "squeegee blade". A squeegee blade holder is mounted to the squeegee assembly, and both the squeegee blade holder and squeegee assembly are positioned above and extend transverse to the circuit board or other substrate horizontally positioned on the tooling plate which is part of the screen or stencil printer.
A squeegee blade is inserted into the squeegee blade holder so that the squeegee blade is superjacent the circuit board and extends transverse thereto. The squeegee head assembly is driven over the screen, stencil, or template positioned between the circuit board and the downwardly-projecting squeegee blade by some type of electric or pneumatic motor or cylinder drive assembly. This movement is referred to as the "squeegee stroke". For most printers the squeegee head assembly can print in both directions (forward or backward), and the movement of each stroke can be individually controlled. In addition, many screen and stencil printers permit x, y, and theta (rotation) adjustments of the circuit board or other substrate placed on the printer. Moreover, some types of printers include features which permit adjustment of both the speed and stroke length of the squeegee head assembly and adjustment of the pressure of the squeegee blade against the material and/or substrate. Metal squeegee blades are typically used in place of polyurethane squeegee blades when a company is printing high viscosity materials, such as solder paste or epoxy, onto printed circuit boards or other substrates.
In operation, the squeegee blade is driven down under pressure until it comes into contact with, or is just above, the top surface of the screen or stencil. With the squeegee blade forced down under pressure, the drive assembly drags the squeegee blade across the stencil. As the squeegee blade travels above the stencil, the solder paste--or other material previously described --is dragged and pushed along the top surface of the stencil by the squeegee blade. The simultaneous downward pressure and forward travel of the squeegee blade against the solder paste forces the paste through the stencil or screen and deposits the solder paste onto the circuit board. The downward pressure and longitudinal movement of the blade forces the solder paste through the stencil or screen apertures and, in the process, wipes clean the surface of the screen or stencil. The bead of solder paste is not dragged across the top surface of the screen or stencil; in fact, the bead of solder paste rolls across the top surface of the screen or stencil like a steam roller traveling over a road surface. The rolling action of the solder paste caused by the downward pressure and forward movement of the squeegee blade is what causes the paste to be pushed and passed through the screen or stencil apertures for deposition onto the circuit board. Once the solder paste is forced through the interstices of the template or the apertures of the stencil, capacitators, resistors, and other electrical components are mounted onto the wet paste deposited on the circuit board to complete the electrical circuitry of the board. Then the circuit board goes through a heating/curing process and the solder paste is transformed into what in the art is called pads and lines which create the electrical circuitry on the circuit board.
Among the critical factors which affect the passing of solder paste through the screen or stencil apertures and deposition of solder paste onto the circuit board is the angle at which the squeegee blade meets the top surface of the screen or stencil. This is referred to in the industry as the "angle of attack". Varying this angle affects the amount of solder paste or other material deposited through the screen or stencil apertures and onto the substrate surface.
The majority of metal squeegee blades currently in use comprise two flat, elongated, aluminum or steel plates between which a flat or bent piece of steel (the actual squeegee blade) is sandwiched. The plates are fastened together to hold the blade in position. This sandwich blade assembly is then inserted into a squeegee holder. The result is a metal squeegee blade whose angle of attack is fixed relative to the stencil or screen over which the blade will pass during the printing process. An alternative is to cut two elongated blocks of steel or aluminum at an angle so that the blade located therebetween is sandwiched at an angle. Moreover, some screen and stencil printers include a feature which permits adjustment of the entire squeegee head assembly so that the angle of attack can be varied. However, this feature is usually not standard on printers but comes as an option and is a separate mechanism from the blade.
Therefore, there is a need for a squeegee blade which incorporates an adjustable "angle of attack" in its design, and is readily adaptable to the various squeegee blade holders and squeegee head assemblies in use in the surface mount technology field.